The present invention relates to transistor electrodes and, more particularly, to a T-shaped transistor electrode and a method of manufacturing it.
For transistor electrodes, such as those used to make contact with transistor emitters, it is often desired for the electrode to have a T-shaped cross-section. The T-shape includes an upper cross-piece portion which is perpendicular to a lower stem portion. One advantage of using a T-shape electrode is that the upper cross-piece portion is usually wide and thus capable of carrying relatively large current, while the lower stem portion is narrow to reduce adverse parasitic effects at the point where the electrode makes contact with the semiconductor substrate.
Another advantage of a T-shaped electrode is that the upper cross-piece portion overhangs the lower stem portion. This overhang permits the electrode to function as a "mask" when depositing neighboring electrodes. For example, a base electrode may be deposited next to a T-shaped emitter electrode. In this scenario the overhanging portion of the upper cross-piece portion of the T-shape functions as a "mask" for one side of the base electrode during its deposition.
Prior art T-shaped electrodes are typically formed on transistor substrates by using at least two mask/metal deposition steps. The first mask and metal deposition step is used to define the stem portion of the T-shape, and the second mask and metal deposition step is used to define the upper cross-piece portion of the T-shape. The use of two mask steps results in a two piece electrode, the lower stem portion being formed from one layer of metal, and the upper cross-piece portion being formed from another layer of metal.
The performance of a heterojunction bipolar transistor can be improved by placing the emitter and base electrodes very close to each other. As will be explained in detail below, however, when prior art T-shaped emitter electrodes are used as "masks" for forming base electrodes, the resulting base electrodes are often shorted with the emitter electrodes. A transistor will not operate if its emitter electrode is shorted with its base electrode.
FIG. 1 illustrates a prior art transistor emitter electrode 20 having a T-shaped cross-section. The electrode 20 is formed on a transistor substrate 26. The emitter electrode 20 includes two separate pieces of electrode metal: an upper cross-piece portion 22 and a lower stem portion 24. The upper cross-piece portion is positioned perpendicular to the lower stem portion. Furthermore, the upper cross-piece portion is ideally centered over the lower stem portion. Two base electrodes 28 and 30 are located in base wells 32 and 34.
The lower stem portion of the T-shaped emitter electrode is deposited on the transistor substrate 26 during a first mask and metal deposition process. The upper cross-piece portion is deposited on the lower stem portion during a second mask and metal deposition process. Each of the mask and metal deposition processes typically requires application of a layer of photoresist, masking of the photoresist, exposure of the photoresist to ultraviolet light, removal of the exposed region, and deposition of metal.
The base wells 32 and 34 extend beneath each side of the upper cross-piece portion, as indicated by reference numerals 36 and 38, to provide clearance between the base and emitter electrodes. The purpose of the extension of the base walls is to permit the upper cross-piece portion to be used as a "mask" when the base electrodes 28 and 30 are deposited in the base wells. Using the upper cross-piece portion as a "mask" results in the right edge of one base electrode 28 being aligned with the left edge of the upper cross-piece portion, and the left edge of the other base electrode 30 being aligned with the right edge of the upper cross-piece portion.
The upper cross-piece portion includes overhang regions 40 and 42 which overhang the lower stem portion. In order for the base electrodes to be positioned properly with respect to the lower stem portion, the overhang regions should be equal in length. The overhang regions will be equal in length when the lower stem portion is positioned in the center of the upper cross-piece portion, or in other words, when the upper cross-piece portion is "aligned" with the lower stem portion. When the upper cross-piece portion and lower stem portion are aligned, the base electrodes 28 and 30 are aligned with respect to the T-shaped emitter electrode.
Thus, it is advantageous for a T-shaped electrode to have the upper cross-piece portion of the T-shape aligned with the lower stem portion of the T-shape. In practice, however, prior art T-shaped electrodes often resemble the emitter electrode 44 shown in FIG. 2 having misaligned upper and lower portions. The two piece emitter electrode includes an upper cross-piece portion 46 and a lower stem portion 48. As shown, the upper cross-piece portion is misaligned with the lower stem portion. In other words, the lower stem portion is not positioned in the center of the upper cross-piece portion. This results in unequal overhang regions of the upper cross-piece portion.
Base wells are usually formed in a substrate subsequent to formation of the emitter electrode. As discussed above, the base wells should extend beneath each side of the upper cross-piece portion. In the transistor shown in FIG. 2, when base wells 50 and 52 are formed in the substrate 54, base well 52 must undercut the lower stem portion 48 in order to underlie the upper cross-piece portion. This undercutting of the lower stem portion is due to the upper cross-piece portion being misaligned with the lower stem portion. When the upper cross-piece portion is used as a "mask" for depositing base electrode 58, the resulting base electrode 58 is shorted with the lower stem portion 48 at point 60.
Another problem caused by the parts of the emitter electrode being misaligned is that the distance between one base electrode 56 and the lower stem portion 48 is greater than the distance between the other base electrode 58 and the lower stem portion. Device performance is decreased when the base electrodes are located unequal distances from the emitter electrode. Device performance is decreased because the conductive channels which are formed in the substrate 54 between each of the base electrodes and the emitter electrode during device operation are not equal in length.
One way to prevent base and T-shaped emitter electrodes from shorting together is to increase the distance between them. However, as the separation between the emitter and base electrodes increases, the device performance decreases. Thus, this is not a desirable solution to the problem.
Thus, there has developed a need for a T-shaped electrode capable of having a neighbor electrode deposited very close by without the two electrodes coming into contact with each other.